1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a mobile communication system having a base station on a manhole cover laid on a road.
2. Description of the Related Art
Up to now, in a base station in the mobile communication system, in order to ensure an electric wave propagation distance, a transmit/receive antenna is located at a position higher in height than the ground where an unobstructed view is got, whereby a service area of the base station is ensured as large as possible, and the planning for placing base stations has been conducted restraining the number of the base stations.
However, the spread of the mobile communication system in the recent years makes the mobile communication system of the micro cell system spread. This leads to the necessity that a large number of base stations that covers small service areas are located so as to effectively utilize the frequency resources.
As a result, it becomes difficult to ensure locations where the base stations and the base station transmit/receive antennas are situated, and there has been demanded a method of locating the base stations and so on in which the base stations and the base station transmit/receive antennas can be located without degrading the beauty of streets.
Under the above circumstances, there has been proposed a system in which a sewerage or an underground multi-purpose duct is used as a transmission path between the switching centers, etc., and the base stations, optical fiber cables and power supply cables are situated in the sewerage, etc., and base station units are assembled with manhole covers provided on the main portions of the sewerage, etc.
For example, Japanese Patent Unexamined Publication No. Hei 5-227073 discloses a structural example of an information communication system in which a transmit/receive antenna and a photo-electric conversion device for conversion between a transmitter/receiver and an optical fiber cable are equipped in an manhole cover laid on a road.
However, the above conventional system suffers from r problems stated below.
FIGS. 11 and 12 are diagrams for explanation of problems with the conventional manhole type base station.
(1) In the transmit/receive antenna and other devices in the manhole type base station, because their located positions are fixed, an electric wave radiation angle of the transmit/receive antenna 111 is fixed as shown in FIG. 11, to thereby make the dimensions of a service area which can be covered by one base station constant.
The location of the manhole per se is determined by the technically required conditions for locating in the sewerage or the underground multi-purpose duct and conditions for maintenance and management in the sewerage or the underground multi-purpose duct.
For that reason, the manhole cannot be located provided that electric waves are radiated from the base station in a certain service area without any gaps. That is, a non-sensitive band is caused to always occur in the service area depending on the conditions such as intervals of the manholes, positions at which the manholes are located, places at which the manholes are located, and so on.
(2) Because the manhole cover per se is laid on the road, there is a case in which a person, a car or so on which passes along the road is put on the manhole or covers the manhole. In this case, the electric wave radiation from the base station is impeded, as a result of which a reduction of the service area, the interruption of communication, and so on occur frequently and rapidly.
In the conventional mobile communication system, a received electric field intensity is measured in the base station that controls the communication operation of the mobile station, and in the case where the received electric field intensity is weakened, the electric field intensities in the peripheral base stations are measured, and hand-off control that switches the operation to a base station having the highest electric field intensity is conducted. However, in the case where a reduction of the service area, the interruption of communication or the like occurs frequently and rapidly as described above, since the received electric field intensity in the base station is measured even after the reduction of the service area, the interruption of communication or the like occurs, a period of time of interruption becomes long, to thereby lead to a trouble such as an error of talking data or an instantaneous interrupt noise in the talking operation.